A great variety of scientific inquiry is confronted with the challenge of identifying the structure or composition of particular substances. To assist in this identification, a variety of schemes have arisen which require the ionization of the particular substance of interest. This need spans all charged particles including subatomic particles, small ions, and charged particles and droplets exceeding a micron in diameter.
In many such ion generating schemes, the presence of a gas or air is either essential to the ionization process or is an unavoidable consequence of the process. For example, in some cases, the ion current is measured, generally as a function of time, to assist in the identification, as in ion mobility analysis, or with thermal, flame or photoionization detectors used in conjunction with gas chromatography separations.
Charged particles beams are also used in ion guns, ion implanters, laser ablation plumes, and various mass spectrometers (MS), including quadrupole MS, time of flight MS, ion trap MS, ion cyclotron resonance MS, and magnetic sector MS. In mass spectrometry applications, typical arrangements often combine the charged particles or analyte with a carrier gas in an electrical field, whereupon particles are ionized by one method or another (e.g., inductive charging of particles) for use in an analytical process.
Many of these analytical techniques, as well as the other industrial uses of charged particles, are carried out under conditions of high vacuum. However, many ion sources, particularly sources used in MS and other analytical applications, operate at or near atmospheric pressures. Thus, those skilled in the art are continually confronted with challenges associated with transporting ions and other charged particles generated at atmospheric or near atmospheric pressures, and in many cases contained within a large gas flow, into regions maintained under high vacuum.
An illustrative example of this general problem is presented in the use of mass spectrometry as an analytical technique. In many applications of mass spectrometry, a charged particle or ion beam is generated at a higher pressure, for example, approximately atmospheric pressure in the case of electrospray ionization, and is then passed to a region maintained at a much lower pressure where the mass spectrometer can function effectively. In such an arrangement, the charged particle beam is directed through at least one small aperture, typically less than 1 mm diameter, which is used to maintain the pressure differential. Several stages of differential pumping are often used to create large pressure differences, and thus each of the regions are connected in series through apertures in order to allow gas flow into the lower pressure region.
Because of the dispersion of the charged particle beam, and the limited cross section defined by the aperture, a significant portion of the beam is typically unable to pass through each aperture and is thus lost. In many applications, a portion of the beam which is lost includes ions of interest, and may thus result in a decrease in the sensitivity of the analytical device. This can serve to preclude many analytical applications. Also, a loss of a portion of the beam may result in a disproportionate loss of the ions of analyte because the ions of analyte may not be evenly distributed throughout the charged particle beam.
In other uses of charged particles, it may be desirable to direct or collect dispersed charged particles which have not been generated as part of an charged particle beam per se. For example, in an atmospheric charged particle sampling device, it may be desirable to sample a large volume of air for the presence of some charged particles of interest. These charged particles may be ambient, or produced by photoionization or other means. It would be useful to have a means by which charged particles in the air are captured and directed to a detector, collector or other devices. Examples of possible uses include environmental monitoring for releases of ambient ions, aerosols, and other ion-producing processes such as combustion.
To assist in the transfer of ions and other charged particles at lower pressures, the use of DC electrical (electrostatic) fields, generated by a variety of methods, for the manipulation of charged particles or to assist in the collection of charged particles, is well known in the art. In ion sources operated at higher pressures, an unavoidable consequence is the presence of gas phase collisions and charge-charge repulsion interactions that lead to expansion of the ion cloud. Conventional ion optics devices such as electrostatic devices, which can function effectively to focus ions under vacuum conditions, are ineffective for avoiding or reversing the ion cloud expansion brought about by gas phase collisions and the repulsive electrical forces between charged particles. Also, time varying (electrodynamic) or radiofrequency (RF) electric fields can be applied for focusing purposes. An example of such RF devices are RF multipole devices in which an even number of rods or "poles" are evenly spaced about a line that defines the central axis of the multipole device. These include quadrupole, hexapole, octopole and "n-pole" or greater multipole devices that are used for the confinement of charged particles in which the phase of the RF is varied between adjacent poles. The use of these devices can result in focusing of an ion beam due to collisional damping in the presence of a gas as described in U.S. Pat. No. 4,963,736 to D. J. Douglas entitled "Mass Spectrometer and Method with Improved Ion Transmission" and U.S. Pat. No. 5,179,278 to D. J. Douglas entitled "Multipole Inlet System for Ion Traps." It is generally recognized that RF multipole devices can be used to trap or confine charged particles when operated at appropriate RF frequencies and amplitudes. In such arrangements, the motion of charged particles of appropriate mass and charge is constrained by the effective repulsion (of the "pseudo potential") arising from the RF field near the electrodes (poles). The charged particles thus tend to be repulsed from the region near the electrodes and tend to be confined to the inner region which is relatively field free. Thus, for example, in quadrupole devices, which are typically operated in high vacuum, ions tend to oscillate within the area inscribed by the four poles. In multipole devices with larger numbers of poles, the increased number of poles enlarges the region of lower field, or region which is effectively field free. Also known in the art are ring electrode devices wherein the field free region is dictated by the diameter and the spacing between the rings. Ring electrode devices consist of conductive rings having approximately equal spacing between rings, and have confinement properties determined by the diameter of and the ring thickness which roughly corresponds to the properties determined by the rod diameter and spacing in multipole devices. The similar alternating phase of the RF voltages for each subsequent ring of such devices enables their use as "ion guides." Such devices are used far less frequently than conventional multipole ion guides.
Also known in the art are quadrupole mass filters which use DC potentials with quadrupole devices to discriminate ions according to their mass to charge ratio. In the absence of the DC potentials and in the presence of a low pressure gas, these types of ion guides do result in a reduction of the dispersion of the ions due to collisional damping of charged particles to the field free region. At higher pressures however, ion velocities may become too small for ions to rapidly exit the multipole, resulting in a build up of space charge and decreased ion transmission.
The nearly field free region is constant across the length of the multipole or ring electrode device and includes some fraction of the volume inscribed by the poles or rings. Given a fixed number of poles or rings, the nearly field free region may thus only be significantly increased by increasing the distance between the poles or rings and the diameter of the poles or rings, both of which require an increase in the RF voltage applied to the poles or rings to obtain effective confinement. Again, given a fixed number of poles or rings, the size of a cross section of the field free region, and thus the size of the region which accepts ions (or the ion acceptance region), increases as the square root of the RF voltage applied to the poles or rings. Thus, to create any significant gain in the cross section of the field free region, and thus the ion acceptance region, in practice requires prohibitively large RF voltages. Larger acceptance regions can also be obtained by the use of higher multipole devices, but a general failing of this approach is that the nearly field free region becomes correspondingly large and effective focusing to a small region is not obtained. Thus, the ability to focus ions through a small diameter aperture is reduced.
U.S. Pat. No. 5,572,035 to Jochen Franzen, entitled "Method and device for the reflection of charged particles on surfaces", describes a variety of configurations of strong but inhomogeneous RF fields of short space penetration for the reflection of charged particles of both polarities at arbitrarily formed surfaces. As described by the inventor, this device "is particularly useful for the guidance and storage of ions in a pressure regime below about 10.sup.-1 millibar, and with frequencies above 100 kilohertz. It may be used at normal air pressures for charged macroparticles." Thus, as described by the inventor, the invention of the Franzen patent is ill suited for operation at pressures close to atmospheric, where the transition from an ion source to an instrument having a low pressure region would be located, except for macromolecules, and only then through the use of audio frequencies. Such macromolecules, or macroparticles, are many orders of magnitude in both mass or mass to charge rations than analyzed by mass spectrometry.
Thus, there exists a need for a device which can both guide ions and focus a dispersion of charged particles at near atmospheric pressures.